Field of the Invention
This invention relates to an improved still suitable for purifying brackish water. More particularly, this invention relates to a plastic heat exchange membrane and method for transferring heat from a condensing vapor on a non-wettable surface on one face of the membrane to a wettable evaporating surface on the opposing face of the membrane. The evaporating surface of the membrane may be made wettable by the application of a corona discharge to the surface, or by the application of a wettable coating to the surface.
Plastic heat exchange membranes to be used in vapor compression and multi-effect stills are known in the art. Water vapor as well as other vapors with polarized molecules tend to condense on plastic films which feature unpolarized molecules and are thereby inherently unwettable, and the water vapor condenses in drop-wise manner instead of in film-wise manner. The primary advantage of drop-wise flow is that when the condensate flows down the surface of the plastic, it leaves behind a `dry` surface which features a significantly higher heat exchange to a condensing vapor than a wetted surface, as described in the prior art.
An unwettable surface has the disadvantage that, on the evaporation side of a heat exchanger, a thin film of distilland is not readily attainable on the evaporation surface due to the distilland `beading` on the surface and leaving `dry` areas on the evaporation surface. Since the dry areas do not take part in the evaporation process this significantly decreases the effectiveness of the evaporator. Certain known evaporators use various screens, meshes, and wicks in an attempt to force the distilland to spread in a controlled manner over the evaporating surface.
Water distillation cycles are based on the interaction of three principles: 1. It takes approximately 1,000 b.t.u.s. of heat to evaporate one pound of water, and such quantities of heat are too expensive to allow direct, single-stage evaporation to be used in plants having large output. Besides, the vapor has the latent heat absorbed by its molecules which must be given up in order for the vapor to condense. 2. The saturation pressure of water vapor varies greatly with small changes in water temperature. Saturation pressure is the absolute pressure of a quantity of vapor which is at its boiling/condensation temperature with no other vapors present. Steam tables are readily available which document this relationship. 3. The saturation pressure of water at a given temperature is reduced if salts are dissolved in the water. For normal sea water, the reduction in saturation pressure is about 1.84%. Thus, pure water at 212 degrees Fahrenheit has a saturation pressure of 14.7 pounds per square inch. However, sea water at the same temperature will have a saturation pressure 1.84% below this, or only 14.4 pounds per square inch. Thus, if a chamber contains sea water and water vapor the space above the sea water, and if the water is 212 degrees Fahrenheit, then the vapor, if it is in equilibrium with the sea water, will be at 14.4 pounds per square inch absolute (psia) pressure instead of the 14.7 pounds per square inch as would be produced with pure water. Conversely, if it is desired to have the vapor at 14.7 psia, the sea water temperature must be increased to 213.03 degrees Fahrenheit, which is higher by 1.03 Fahrenheit degrees than is needed to maintain the same pressure with pure water. These distillation cycles require all non-condensible gasses to be removed from process equipment and from the supply brine in order to work efficiently. Hence, distillation processes normally take place under pressure or in a vacuum, but rarely at ambient atmospheric pressure.
A simple distillation cycle may include a number of stages that are thermally connected in series, with a hot source at one end and a cold source at the other end of the series. Each stage has its own supply of brine, which flows over an evaporator. Each stage also has a condensor which is in heat-exchange relationship with the evaporator of the adjacent, lower temperature stage. The temperature difference between each stage is approximately equal to the total number of stages divided by the difference in temperatures of the hot source and the cold source at opposite ends of the series of stages. The operation and the thermal flow may be described as follows: In the first stage the hot source heats the evaporator in that stage which, in turn, vaporizes some of the brine flowing across the evaporator. The vapor is then condensed at the condensor for the stage, and as the vapor condenses, its latent heat energy is transferred to the condensor and then to the evaporator of the next stage which, in turn, vaporizes a portion of the brine flowing across its surface. This procedure is repeated for each stage until, at the last stage, the heat from the condensor is transferred into the cold source. The advantage of a multiple-stage still is that the water produced from evaporating the brine in the first stage is increased by all of that produced from the additional stages without any significant additional increase in energy.
A vapor compression cycle may include a single stage evaporator to vaporize the brine within an evaporation chamber, a single stage condensor to condense vapor in a condensation chamber, and a vapor compressor to raise the pressure and therefore the temperature of the vapor as the vapor passes from the evaporation chamber into the condensation chamber by operation of the compressor. The condensor and evaporator are in heat-exchange relationship with each other, such that heat can flow from the condensor to the evaporator. In operation, incoming brine flows over the evaporator and a portion of the brine is evaporated. The vapor from the brine is supplied to the compressor where its pressure and temperature are increased, and from which it is then exhausted into the condensation chamber. The increase in pressure should be sufficient to assure that the saturation temperature of the compressed vapor is higher than the boiling point temperature of the distilland flowing over the evaporator, and is sufficiently higher to provide an acceptable rate of heat flow from the condensor to the evaporator. The advantage of the vapor-compression cycle is that most of the heat required to evaporate the brine is supplied by the condensation of the vapor, thereby reducing the primary energy requirement substantially to the energy needed to compress the vapor.
A multi-stage flash distillation process is similar to the multi-stage process previously described in that a number of stages are inserted between a hot source and a cold source. Each stage includes a separate pressure/vacuum chamber, a condensor to condense vapor from within the chamber, and an open channel within the chamber through which brine can flow so that vapor which is given off by the brine can flow freely to the condensor to be condensed and collected. In this process the brine is heated at the first stage to the temperature of the hot source. It then flows in an open channel from one stage to the next until it reaches the final stage. Each stage is at a lower pressure and temperature than its preceeding stage so that as the hot brine flows into a stage, it will be at a higher temperature than the saturation temperature of the stage and will evaporate explosively (flash) until sufficient vapor is evaporated to reduce the brine to the saturation temperature of the stage. This happens throughout the stages until, by the time the flowing hot brine reaches the last stage, it has given up most of its latent heat. In the meantime, cold brine is transported from the last stage to the first, typically within heat exchanging tubes. The cold brine is colder than the saturation temperature of a stage as the cold brine begins flowing through the heat exchanging tubes for that stage, with the consequence that condensation typically takes place on the outside of the heat exchanging tubes and the latent heat of condensation is transferred from the vapor as it condenses, across the walls of the heat-exchanging tubes, to heat the cold brine as it flows within the heat-exchanging tubes for the stage. By the time the cold brine passes through the first or hottest stage (its final stage), it has been heated almost to the temperature of hot brine and an external heating source makes up the difference in temperature.
The condensor within any given stage of a multi-stage flash distillation system is similar to a condensor suitable for use in a steam power plant that operates a steam engine or turbine. In such instances, a cooling liquid flows through the interior of a condensing tube and heat exchange takes place through the walls of the condensing tube with the vapor to be condensed, and if it is unimportant whether the vapor to be condensed came from a flashing brine or from the exhaust of a steam turbine.
Ambient pressure diffusion distillation operates on different principles from the previous methods discussed. In this process, a number of cells are used, similar to the multiple-stage still previously described in that each cell includes an evaporator and a condensor. The first cell is at the highest temperature and has an external heat source to heat its evaporator. The last stage is at the lowest temperature and has an external cold source to extract heat from its condensor. Between the first and last cells, the evaporator of one cell shares a common wall with the condensor of the preceeding cell and is in heat-exchange relationship with it. However, the cells are not evacuated but rather are allowed to remain at ambient atmospheric pressure through the use of small ports which allow air to flow into the cells. The vapor then does not flow directly from the evaporator to the condensor of the cell, but instead diffuses through the air-vapor mixture. Diffusion is much slower than direct or forced flow, and the condensate produced per square foot of heat exchanger surface area is correspondingly less. However, the surface area of the heat exchangers may be increased to compensate for the lower performance.